The present invention relates generally to dielectric filters, and, more particularly, to a dielectric filter construction having at least two resonators of trapezoidal-shaped cross-sections.
Advancements in the field of radio electronics have permitted the introduction and commercialization of an ever-increasing array of radio communication apparatus. Advancements in electronic circuitry design have also permitted increased miniaturization of the electronic circuitry comprising such radio communication apparatus. As a result, an ever-increasing array of radio communication apparatus, comprised of ever-smaller electronic circuitry, has permitted the radio communication apparatus to be utilized more conveniently in an increased number of applications.
A radio transceiver, such as a radiotelephone utilized in a cellular, communication system, is one example of radio communication apparatus which has been miniaturized to be conveniently utilized in an increased number of applications. Additional efforts to miniaturize further the electronic circuitry of such radio transceivers, as well as other radio communication apparatus, are being made. Such further miniaturization of the radio transceivers will further increase the convenience of utilization of such apparatus, and will permit such apparatus to be utilized in further increased numbers of applications.
Pursuant to such efforts to miniaturize further the electronic circuitry comprising radio transceivers, as well as other radio communication apparatus, size minimization of the electronic circuitry comprising such is a critical design goal during circuit design of such circuitry.
Dielectric block filters, comprised of a ceramic material, frequently comprise a portion of the circuitry of such radio transceivers. Such dielectric block filters are advantageously utilized for the reason that such filters exhibit good filter characteristics at frequencies (typically in the megahertz and gigahertz range) at which such transceivers usually are operative.
To form a filter of a block of dielectric material, holes are formed to extend through the block, typically to extend between top and bottom surfaces of the block. More particularly, such holes form cylindrical cavities which extend between such top and bottom surfaces of the block. Sidewalls defining such cylindrical cavities are coated with an electrically-conductive material, such as a silver-containing material.
Typically, substantial portions of the outer surfaces of the dielectric block are similarly coated with the electrically-conductive material. Portions of a top surface of the dielectric block are also typically coated with the electrically-conductive material. Uncoated portions of the top surface of the dielectric block separate portions of the top surface of the dielectric block which are coated with the electrically-conductive material. Adjacent portions of the top surface of the dielectric block coated with the electrically-conductive material form capacitive plates which are separated by such uncoated portions. Pairs of capacitive plates, so formed, form capacitors thereby.
Because of the coating of the electrically-conductive material upon the sidewalls which define the cavities extending through the dielectric block, such cavities form resonators which resonate at frequencies determined by the lengths of such cavities through the dielectric block. Electromagnetic coupling between adjacent ones of the resonators formed thereby (and, to a lesser extent, between nonadjacent ones of the resonators), and the capacitors formed of the top surface of the dielectric block together define a filter having filter characteristics for filtering a signal to such dielectric filter. Appropriate selection of the component values of the filter formed thereby is, of course, determinative of the precise filter characteristics of such filter. The component values of the capacitors formed of the painted portions formed upon the top surface of the dielectric block may be altered by altering two parameters. First, the spacing between the coated portions of the top surface of the dielectric filter and the dimensions of such coated portions may be altered. Second, the spacings between such cavities as well as the amount of electrically-conductive material coated upon the sidewalls defining such cavities may be altered, also to alter the amount of electromagnetic coupling between adjacent ones of the resonators.
Historically, the component values of the elements comprising the filter have been selected by altering the two above-noted parameters. Namely, the capacitive values of the capacitive elements disposed upon the top surface of the dielectric filter have been altered by changing the physical dimensions of the coated portions forming the capacitive plates on the top surface and altering the spacing between such capacitive plates. And, spacings between adjacent ones of the resonators have been altered.
Alteration of the capacitive values of the capacitive elements formed upon the top surface of the dielectric block has increasingly become an impractical means for altering the filter characteristics of the dielectric filter. As the physical dimensions of the dielectric filters are reduced, the surface area of the top surface of such dielectric filters are correspondingly reduced. The capacitive values of the capacitive elements are dependent upon the physical dimensions of the coated portions of the top surface as well as the respective spacings between such coated portions; the reduction in the surface area of the top surface of the dielectric filter limits the dimensions of the capacitive plates of the capacitive elements as well as the spacings between such capacitive plates. As a result, the range of capacitive values of which the capacitive elements formed therefrom is reduced. Accordingly, alteration of the filter characteristics of a dielectric filter has become increasingly limited.
Similarly, alteration of the spacings between the adjacent ones of the resonators formed of the cavities extending through the dielectric filter has also increasingly become an impractical means of altering the filter characteristics of the dielectric filter. As the physical dimensions of the dielectric filter are reduced, permissible spacings between adjacent ones of the resonators are of increasingly smaller maximum distances. As a result, the amounts of couplings between adjacent ones of the resonators are of increasingly limited ranges of values. Accordingly, alteration of the filter characteristics of the dielectric filter in such a manner has become increasingly limited.
By altering the dimensions of the cavities forming the resonators and by altering the dielectric material separating adjacent ones of the resonators, the electromagnetic couplings between adjacent ones of the resonators may be altered. For instance, rather than forming the cavities to be cylindrical in dimension, the sidewalls defining such cavities may be of elliptical cross-sectional configurations. Alteration of the dielectric material separating the adjacent ones of the resonators may be effectuated by forming notches along side surfaces of the dielectric filter between the adjacent ones of the resonators. In such instances, air rather than the ceramic material of the dielectric filter comprises portions of the dielectric material separating the adjacent ones of the resonators. When such notches are formed to extend too deeply into the side surfaces of the dielectric filter, the structural integrity of the filter may be compromised. Accordingly, alteration of the filter characteristics of a dielectric filter in such a manner may also be somewhat limited due to the need to maintain the structural integrity of the dielectric filter.
Accordingly, what is needed is a dielectric filter construction, and means for making such, which permits component elements thereof to be selected over a wide range of values without requiring increase in the physical dimensions of the filter.